If you are new to my blog, or haven't been reading on Thursdays, this is a segment where I post my thoughts about various nutrition and health related topics, or maybe even something else if it is really an intriguing topic. Most of my current views and opinions come from experience working as a dietitian and as college instructor in nutrition courses. Most of that learning is science based or observational from the clinical setting. In general, I hate overgeneralizations, so for many of the things I post, I expect there is one or two exceptions because broad sweeping generalizations are always bound to miss out on something.

This week I am going to keep it short and sweet...and to the point. Some of you may have noticed I have been a little busy recently. I have been able to read some blogs, but I am having trouble keeping up with commenting on all the great posts. Please forgive me! I hope to have more time soon. I am nominating committee chair and I am spending much of my time getting candidates together for the ballot and this all needs to be ready in just under a month.

This week I want to talk a little about BMI. I know so many people hear this term and are not really sure what it really means. I actually found this defined entirely wrong in a textbook that my students use. I have no idea how the authors messed that one up so badly.

1) BMI is Body Mass Index and it is a ratio of a person's weight to their height. This is more appropriate for assessment than comparing person A to person B. There may be many variables, so comparing two people may not give a clear picture. Instead, BMI is used to assess the body weight in relationship to the body height. The problem with this is that not all parts of the body weight are the same. Muscle and fat are very different and excess fat may have a negative effect on the body, but an increase in lean muscle mass does not. In fact, increased lean muscle mass helps the metabolism to perform better.

2) My biggest issue with BMI and the implications the labels have on body weight is that lack of differentiation between males and females. The HAMWI method for calculating a desirable body weight accounts for males and females, so when a % desirable body weight is calculated for a patient, client, or who ever, a male may have a low %DBW, but the BMI is slightly inflated so that the person seems to be in a healthy weight. Same goes for a person with a healthy BMI, they may come up as overweight based on their %DBW. The numbers match up for females, or so it seems, but not when looking at males. Most of the time the numbers make sense, but sometimes they really don't and it is hard to assess risk. For me, this is the biggest problem when a BMI indicates healthy weight, but they are considered at risk based on their %DBW.

3) A new concern I have with BMI is that it seems to just be a label and used for insurance purposes in some cases, and a person may not have any idea what the number really means. Some people may get away from monitoring the number on the scale, but this is just another thing for people to monitor. Monitoring is great, but it is important to understand what the numbers mean. There are specific categories, but sometimes I feel like they are a label. Almost like a clinician might refer to a person by this label rather than addressing them in light of a bigger picture. BMI tells part of the story, not the whole story. Treating someone based on a weight scale label should not be an approached used. I am not sure how often this happens, but I have seen some cases where the treatment was not tailored to the patient, but rather tailored just to their weight.

4) That brings me to the next point. As I said, weight does not mean fat. These are not equal. I already mentioned muscle at the start. If someone is an athlete and working on building muscle mass, and the BMI indicates they are overweight or obese, the persons health needs to be addressed differently than a person with the same BMI but much less muscle mass. Telling a person they need to lose weight is only appropriate when it accounts for all factors. Yes, weight loss helps in almost all cases when it comes to BMI, but health is not 100% linked to weight alone. BMI is only part of the big picture.

5) Oh boy, that brings me to my last point. In my opinion, BMI does 1 thing, and 1 thing only. It is a value that a clinician can use to indicate chronic disease risk. Those with a BMI indicating overweight or obese should then be further screened. BMI is a trigger and indicator of risk. Once this is determine and it is determined there is a risk, all the other factors need to be evaluated. A BMI greater than 25 can let a practitioner know this is a person that needs further testing, monitoring, and education about diet and exercise. An elevated BMI might mean test this person for heart issues or diabetes. It means this person needs to be vigilant of their health because they are at risk, not they will get a disease, but at risk. This is where the intervention takes place. BMI is an indicator of risk. Plus, a low BMI also indicates risk, but more with undernutrition (which is malnutrition, just as obesity is malnutrition) and chance for infection.

Overall, a BMI in the healthy weight range does not mean you are free and clear of chronic disease risk. Someone can be at a healthy weight, eat junk food all day long, and have heart disease, but the BMI never indicated that risk. So, do not assume that a healthy BMI means perfect health. It is less likely than a BMI greater than 25, but certainly not impossible. Also, if you are at the lower end of the healthy range, do not assume it is "healthy" to have a little weight gain even if you remain in the healthy range. For some people this could be a gain of 10-20#s and there is nothing healthy about the gain. If you are at the high end of the range and do not want to consider losing weight to be closer to the lower end, keep in mind that closer to the low end means decreased health risk, plus a better chance of not going over a BMI of 25 when that oh so likely weight gain with aging occurs. It is really all about maintenance. Find a weight that is healthy and comfortable for you. Do not focus so much on the numbers, focus more on your health and way your body can best provide that health for you.

5 comments:

I always like your Thursday thoughts! I agree that the BMI is not a "perfect" number. There are many flaws... I weighed pretty much the same I weigh now a few years ago, but my body composition was very different (more fat, less muscle). Also, my cardiovascular fitness level wasn't very good. So yes, the BMI is certainly not a number one should rely on.

I do think BMI can be good for lots of people, but then I look at someone like my husband. Yes, he could lose a few pounds. But he can also ride his bike faster than anyone I know and he's running his first 5k on Saturday. But yet his BMI says he's obese--when anyone who actually looks at him sees he's short with a huge barrel chest and broad shoulders. So it's definitely not perfect!

I agree with you completely-it doesn't even take 'race' into consideration. Asians are a smaller race-and for years we felt that the BMI ranges were too high for Indians-now thankfully the WHO has approved the reduced BMI for Indians/Asians.I had done a post on it way back in 2009:http://bonnenutrition.blogspot.com/2009/02/new-bmi-guidelines-for-indianssouth.htmlIt's still good for a 'guesstimate' of where you stand :)